Semiconductor light emitting device and method for manufacturing the same

ABSTRACT

Certain embodiments provide a method for manufacturing a semiconductor light emitting device, including: providing a first stack film on a first substrate, the first stack film being formed by stacking a p-type nitride semiconductor layer, an active layer having a multiquantum well structure of a nitride semiconductor, and an n-type nitride semiconductor layer in this order; forming an n-electrode on an upper face of the n-type nitride semiconductor layer; and forming a concave-convex region on the upper face of the n-type nitride semiconductor layer by performing wet etching on the upper face of the n-type nitride semiconductor layer with the use of an alkaline solution, except for a region in which the n-electrode is formed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2010-46883 filed on Mar. 3, 2010in Japan, the entire contents of which are incorporated herein byreference.

FIELD

Embodiments described herein relate to a semiconductor light emittingdevice and a method for manufacturing the semiconductor light emittingdevice.

BACKGROUND

To achieve high efficiencies and high outputs, nitride semiconductorlight emitting devices (hereinafter also referred to as LEDs (LightEmitting Diodes)) designed for white lighting devices are being improvedin crystalline structures and device structures, and higher internalquantum efficiencies and higher light extraction efficiencies are beingrealized.

Where InGaN-based crystals are grown, a sapphire substrate is oftenused, because it is inexpensive and stable. A crystal growth with highcrystallinity can be performed on a sapphire substrate with alow-temperature buffer. However, being an insulator, a sapphiresubstrate does not have conductive properties and is low in thermalconductivity. Therefore, electrodes cannot be formed on the back faceside of a sapphire substrate, and p- and n-electrodes need to be formedon the nitride semiconductor side. Therefore, the tendency to causehigher series resistance and the low heat release properties during ahigh-power operation become problems in achieving even higherefficiencies and outputs.

A thin-film InGaN-based LED is known as one of the LED structures thateliminate the above problems and improve luminous efficiencies andoutputs. Such a thin-film InGaN-based LED transfers LED structuralcrystals grown on a sapphire substrate onto another supporting substratesuch as a Si substrate, a copper substrate, or a gold substrate. Asdevices are formed after the transfer onto a supporting substrate havingconductive properties and high thermal conductivity, the current spreadbecomes larger by vertical energization, and the electric conductiveproperties are improved. Further, the heat release properties are alsoimproved.

Also, by forming a structure that has an n-layer as an upper facethrough a transfer and extracts light from the n-layer side, atransparent electrode for diffusing current becomes unnecessary for then-layer having lower resistance than a p-layer. Since light is notabsorbed by a transparent electrode, the light extraction efficiencybecomes higher. This process of transfer includes a process to bondcrystals (epitaxial crystals) formed through an epitaxial growth to thesupporting substrate, and a lift-off process to detach the epitaxialcrystals from the sapphire substrate. The bonding process may involve aplating technique or a joining technique utilizing weight and heat, andthe lift-off process may involve a laser lift-off technique utilizingthermolysis of an interface caused by a laser or a chemical lift-offtechnique.

In such a thin-film LED structure, the difference in refractive indexbetween the surface of a GaN substrate and the external air is as largeas 2.5 times where only a laser lift-off process has been carried out,and the light reflection from the boundary face lowers the lightextraction efficiency.

To counter this problem, a technique of producing concavities andconvexities on the surface of a chip has been suggested. The concavitiesand convexities are formed by regrowing, polishing, and etching ann-type nitride semiconductor layer. According to a method for simpleformation, concavities and convexities are formed by roughening thesurface through alkaline etching performed on the n-layer on the upperface of a GaN substrate on a supporting substrate. In this manner, thelight extraction efficiency is made higher.

By the conventional alkaline etching, however, the sizes of theconcavities and convexities cannot be made larger in many cases, eventhough the entire film thickness is reduced by prolonging the etchingperiod. Therefore, there is a demand for an etching control method bywhich such concavities and convexities as to improve the lightextraction efficiency can be formed on the surface of a GaN layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) through 1(c) are cross-sectional views showing proceduresfor manufacturing semiconductor light emitting devices according to anembodiment;

FIGS. 2( a) and 2(b) are cross-sectional views showing procedures formanufacturing semiconductor light emitting devices according to theembodiment;

FIGS. 3( a) through 3(c) are cross-sectional views showing proceduresfor manufacturing the semiconductor light emitting devices according tothe embodiment;

FIGS. 4( a) and 4(b) are electron micrographs of sections in thevicinities of the surfaces having concavities and convexities formedthereon in semiconductor light emitting devices according to theembodiment and a comparative example;

FIGS. 5( a) and 5(b) are electron micrographs of the surfaces ofsemiconductor light emitting devices according to the embodiment in acase where the etching period is varied;

FIG. 6 is a plan view of the supporting substrate prior to the divisioninto respective devices;

FIG. 7 is a cross-sectional view of a semiconductor light emittingdevice according to the embodiment; and

FIG. 8 is a graph showing the etching period dependencies of theemission intensities of the semiconductor light emitting devicesaccording to the embodiment and the comparative example.

DETAILED DESCRIPTION

Certain embodiments provide a manufacture method including forming ann-electrode is formed on the upper face of the n-type nitridesemiconductor layer of each device, and performing wet etching on theupper face of the n-type nitride semiconductor layer with the use of analkaline solution, to form concavities and convexities during theformation of optical semiconductor devices.

A semiconductor light emitting device according to the embodiment ischaracterized in that a concave-convex region is formed in the surfaceof an n-type nitride semiconductor layer, except for the region where ann-electrode is formed, and in the concave-convex region, firstconcavities and convexities having height differences of 1 to 3 μmcoexist with second concavities and convexities that have heightdifferences of 300 nm or less and are smaller than the first concavitiesand convexities.

The following is a detailed description of an embodiment, with referenceto the accompanying drawings.

Referring to FIGS. 1( a) through 4, a method for manufacturingsemiconductor light emitting devices according to an embodiment isdescribed. FIGS. 1( a) through 3(c) show the procedures formanufacturing semiconductor light emitting devices according to thefirst embodiment.

First, nitride semiconductor layers are sequentially grown on asubstrate (a wafer) for growing nitride semiconductor crystals or asapphire substrate 10 by metal organic chemical vapor deposition(MOCVD), for example. More specifically, a GaN layer 12 to be a bufferlayer, an n-type GaN layer 14, an active layer 16 of a multiquantum wellstructure made of InGaN, and a p-type GaN layer 18 are sequentiallygrown in this order on the sapphire substrate 10 (FIG. 1( a)).

P-electrodes (reflecting contact electrodes) 20 are then formed withstack films of Ni and Ag on the p-type GaN layer 18 (FIG. 1( b)). Thep-electrodes 20 are formed for respective semiconductor light emittingdevices. An adhesive metal film 22 having Ti, Pt, and Au films that areto serve as adhesive metals and are stacked in this order is formed overthe nitride semiconductor crystal films 12, 14, 16, and 18, to cover thep-electrodes 20 (FIG. 1( b)). With this arrangement, the portions of theadhesive metal film 22 in the regions where the p-electrodes 20 areformed are turned into convex portions, and the portions of the adhesivemetal film 22 in the regions where the p-electrodes 20 are not formedare turned into concave portions (FIG. 1( b)). Patterning is thenperformed on the adhesive metal film 22 by a known lithographytechnique. After that, patterning is further performed on the stack film(the nitride semiconductor crystal films) including the p-type GaN layer18, the active layer 16, the n-type GaN layer 14, and the GaN layer 12(FIG. 1( c)).

Through the patterning, the nitride semiconductor crystal films on thewafer are turned into a mesa having a tapered shape in cross-section,with the area of the film plane gradually increasing from the area ofthe film plane of the p-type GaN layer 18 to that of the GaN layer 12.Here, the “film plane” means the upper plane of each of the layers. Whenpatterning is performed on the stack film, a patterned adhesive metalfilm may be used as a mask. Alternatively, patterning may be performedon the stack film before the adhesive metal film 22 is formed, and afterthe patterning, the adhesive metal film 22 may be formed.

Meanwhile, an Au—Sn layer 32 to be an adhesive metal film is formed on aSi substrate 30 to be a supporting substrate (FIG. 2( a)). The adhesivemetal film 22 on the sapphire substrate 10 and the adhesive metal film32 on the Si substrate 30 are placed to face each other, and pressure isapplied to them at a high temperature of 250° C. or higher over acertain period of time, so that the adhesive metal film 22 on thesapphire substrate 10 and the adhesive metal film 32 on the Si substrate30 are bonded to each other. In this bonding, the contact electrodes 20are buried into the adhesive metal film 32, since the melting-pointtemperature of the contact electrodes 20 is much higher than themelting-point temperature of the adhesive metal film 32 (FIG. 2( a)).

As shown in FIG. 2( b), pulse irradiation is then performed with a UV(Ultra-Violet) laser or a KrF laser of 248 nm in wavelength from theside of the sapphire substrate 10, for example, so as to detach thesapphire substrate 10 from the nitride semiconductor crystal films 12,14, 16, and 18. The surface of the GaN layer 12 exposed at this point isthe surface to be subjected to wet etching.

Patterning is then performed on the nitride semiconductor crystal films12, 14, 16, and 18 by a known lithography technique, to divide thenitride semiconductor crystal films 12, 14, 16, and 18 intosemiconductor light emitting devices (FIG. 3( a)). At this point,patterning is not performed on the adhesive metal film 22, and theadhesive metal film 22 is left exposed among the nitride semiconductorcrystal films divided into the semiconductor light emitting devices. Thepatterned nitride semiconductor crystal films are turned into mesas eachhaving a tapered shape in cross-section, with the area of the film planegradually increasing from the area of the film plane of each GaN layer12 to that of each p-type GaN layer 18.

A SiO₂ film 40 as a protection film is then formed to cover the surfacesof the nitride semiconductor crystal films of a tapered shape and theexposed adhesive metals 22 and 32, for example (FIG. 3( b)). The nitridesemiconductor crystal films form mesa structures, the minimum diameterof each lower face of the nitride semiconductor crystal films in contactwith the adhesive metal film 22 is smaller than the minimum diameter ofthe upper face of the adhesive metal film 22, and the minimum diameterof the lower face of the adhesive metal film 22 in contact with theadhesive metal film 32 is smaller than the minimum diameter of the upperface of the adhesive metal film 32. Accordingly, the adhesive metal film22 is in tight contact with the peripheral end region of each lowerportion of the nitride semiconductor crystal films each having a mesashape, and the protection layer 40 without a step separation can beformed, without a void formed between the protection layer 40 and theadhesion metal films 22 and 32.

The protection layer 40 covering the upper face of each semiconductorlight emitting device is then removed. However, the protection layer 40remains on the outer circumferential region of the upper face of eachsemiconductor light emitting device (the upper face of each GaN layer12). With this arrangement, the upper face of each semiconductor lightemitting device is exposed, except for the outer circumferential regionof each upper face (FIG. 3( c)). At this point, the surface roughness ofthe upper face of each GaN layer 12 is approximately in the range of 100nm to 3000 nm.

N-electrodes 44 are then formed at the center portions of the exposedupper faces of the GaN layers 12 (FIG. 3( c)). The n-electrodes 44 maynot necessarily be formed at the center portions, but may be formedanywhere on the exposed upper faces of the GaN layers 12. As thematerial of the n-electrodes 44, it is preferable to use analkali-resistant electrode material. It is particularly preferable touse a material containing one of the following metals: Pt, Au, Ni, andTi. By using such a material, the sizes (height differences) of theconcavities and convexities formed in the upper faces of the GaN layers12 by the later described alkaline etching can be made larger. Since then-electrodes 44 are formed on the flat GaN layers 12, excellent adhesionproperties can be achieved.

After the n-electrodes 44 are formed, an alkali solution is supplied,and wet etching is performed on the exposed upper faces of the GaNlayers 12. In this manner, the exposed upper faces of the GaN layers 12are roughened, or the GaN layers 12 are turned into GaN layers 12 a eachhaving concavities and convexities formed in its exposed upper face(FIG. 3( c)). As the wet etching is performed after the n-electrodes 44are formed, the etching efficiency becomes higher, and deeperconcavities and convexities can be obtained. This is supposedly becauseelectrons or holes travel between the surfaces of the GaN layers 12 andthe n-electrodes 44, and an electrochemical reaction is caused in eachsurface, accelerating the etching. One n-electrode 44 is formed for eachone optical semiconductor device. Accordingly, larger concavities andconvexities are obtained, compared with a case where one electrode isprovided for one wafer. Also, a uniformly etched state can be achievedin each device in the plane of a wafer.

In this embodiment, a potassium hydroxide solution of 1 mol/l in densityand 70° C. in temperature is used as the alkaline solution, and etchingis performed for 15 minutes. As the etching smoothly progresses, thesurface becomes clouded. While being immersed in the potassium hydroxidesolution, the concavities and convexities are exposed to UV rays, andare made even larger as a result. The sizes of the concavities andconvexities are several hundreds of nanometers to several micron meters.The concavities and convexities can also be made larger by performingetching while applying a voltage of 3 to 10 V intermittently between then-electrodes 44 and the GaN layers 12.

FIG. 4( a) shows an electron micrograph of a section of the surface of aGaN layer 12 having concavities and convexities formed in the abovedescribed manner. As can be seen from FIG. 4( a), the concavities andconvexities vary in size. When the heights of (or the height differencesamong) the concavities and convexities formed in the GaN layer 12according to this embodiment are observed in surface and cross-sectionalimages through electron microscopy images, large concavities andconvexities of several micron meters (1 μm to 3 μm) coexist withsmall-order concavities and convexities of several hundreds ofnanometers (300 nm or less). Accordingly, the reflection from theboundary surface between each GaN layer 12 and the air becomes smaller,and the light extraction efficiency can be made higher.

As a comparative example of this embodiment, semiconductor lightemitting devices are formed in the same manner as in this embodiment,except that etching is performed on the upper face of each GaN layer 12without the formation of the n-electrodes 44. FIG. 4( b) shows anelectron micrograph of a section of the surface of the GaN layer 12having concavities and convexities in a semiconductor light emittingdevice of this comparative example. As can be seen from FIGS. 4( a) and4(b), the concavities and convexities formed in the semiconductor lightemitting device of this embodiment are larger than the concavities andconvexities formed in the semiconductor light emitting device of thecomparative example. When the heights of the concavities and convexitiesformed on the GaN layers 12 according to the comparative example aremeasured, only small concavities and convexities of approximately 200 nmare observed, and large-order concavities and convexities of 1 μm seenin this embodiment are not observed.

FIGS. 5( a) and 5(b) show electron micrographs of the surfaces of GaNlayers 12 having concavities and convexities formed thereon in a casewhere etching is performed for five minutes and in a case where etchingis performed for fifteen minutes by the manufacture method according tothis embodiment. As can be seen from FIGS. 5( a) and 5(b), largeconcavities and convexities coexist with small concavities andconvexities when the etching is performed for fifteen minutes as in thisembodiment, but only small concavities and convexities are formed whenthe etching is performed for approximately five minutes.

FIG. 6 is a plan view of semiconductor light emitting devices seen fromthe side of the n-electrodes 44 after the concavities and convexitiesare formed. As can be seen from FIG. 6, undivided devices are placed onthe Si substrate 30. After that, a p-electrode 46 is formed on the faceof the silicon substrate 30 on the opposite side from the side on whichthe n-electrodes 44 are formed, as shown in FIG. 3( c).

After the procedures shown in FIG. 3( c) are completed, dicing isperformed on the protection layer 40, the adhesive metal films 22 and32, and the Si substrate 30, to divide them into respectivesemiconductor light emitting devices. In this manner, the semiconductorlight emitting device shown in FIG. 7 is completed. The semiconductorlight emitting devices of the above described comparative example arealso divided by dicing.

The light extraction efficiency of the semiconductor light emittingdevices of this embodiment manufactured in the above described manner is1.2 times higher than the light extraction efficiency of thesemiconductor light emitting devices of the comparative example, asshown in FIG. 8. FIG. 8 is a graph showing the fluctuations of theemission intensities of the semiconductor light emitting devices of thisembodiment and the comparative examples observed in a case where theetching period is varied.

As described so far, this embodiment can provide a method formanufacturing semiconductor light emitting devices that have high lightextraction efficiency.

In this embodiment, part of the upper face, the side faces of thenitride semiconductor crystal films, and the bonding portions betweenthe side faces and the adhesive metals are covered with the protectionlayer 40 without a step separation. Therefore, even where the upper faceof the nitride semiconductor crystal films is roughened with the use ofan alkaline solution, the active layer and the reflecting contactelectrodes 20 can be thoroughly protected. Accordingly, reflectingcontact electrodes 20 each having a large area can be formed, and thereflectivity can be made higher. Furthermore, a decrease in operatingvoltage can be expected, since large contact electrodes can be formed.Also, since the protection layer 40 without a step separation is formed,leakage and short-circuiting in devices due to metal adherence or thelike during the manufacturing procedures can be prevented. Further,since the protection layer 40 without a step separation is formed, theprocess to manufacture thin-film semiconductor light emitting devicescan be tolerated, even though the process involves intensified impactsfrom the bonding and the laser lift-off technique, for example. Also,cracks and the like are not formed in the protection layer 40.

The supporting substrate may be a silicon substrate, a silicon carbidesubstrate, a substrate formed by bonding germanium to a siliconsubstrate, or a substrate formed by plating a silicon substrate with ametal such as copper. The silicon substrate may be a substrate that hasa plane orientation of (111), (110), or (100), and also has an offangle.

As for the protection layer, it is preferable to use a material thatcontains silicon dioxide, silicon nitride, zirconium oxide, niobiumoxide, or aluminum oxide.

The chemical solution used in the alkaline etching may betetramethylammonium hydroxide, other than potassium hydroxide. Thedesired density here is 0.1 mol/l to 10 mol/l.

As the contact electrodes 20, it is desirable to use aluminum, otherthan silver.

The adhesive metal film 22 preferably contains titanium, platinum, gold,or tungsten.

As the adhesive metal film 32, it is possible to use a low-melting-pointmetal that is a metal eutectic such as Au—Si, Ag—Sn—Cu, or Sn—Bi, or anon-solder material such as Au, Sn, or Cu, other than Au—Sn.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

1. A semiconductor light emitting device comprising: a substrate havinga first face and a second face opposed to the first face; a first metallayer having a lower face facing to the first face of the substrate andan upper face; a stack film including a p-type nitride semiconductorlayer having a lower face facing to the upper face of the first metallayer and an upper face, an active layer provided on the upper face ofthe p-type nitride semiconductor layer and including a multiquantum wellstructure of a nitride semiconductor, and an n-type nitridesemiconductor layer having a lower face facing to the active layer andan upper face, an n-electrode provided in a first region of the upperface of the n-type nitride semiconductor layer; a p-electrode providedon the second face of the substrate; and a concave-convex region formedon a second region of the upper face of the n-type nitride semiconductorlayer, the second region being outside the first region, theconcave-convex region having first concavities and convexities thatcoexist with second concavities and convexities that are smaller thanthe first concavities and convexities, the first concavities andconvexities having height differences of 1 to 3 μm, the secondconcavities and convexities having height differences of 300 nm orsmaller.
 2. The device according to claim 1, wherein the n-electrodecontains one of Pt, Ni, Au, and Ti.
 3. The device according to claim 1,wherein the stack film includes a tapered shape in cross-section, withan area of a film plane gradually increasing from the n-type nitridesemiconductor layer toward the p-type nitride semiconductor layer, thesemiconductor light emitting device further comprises: a contactelectrode provided in a third region of the lower face of the p-typenitride semiconductor layer; a second metal layer having a lower facefacing to the upper face of the first metal layer and an upper facefacing to the lower face of the p-type nitride semiconductor layer, thesecond metal layer covering the contact electrode and being in contactwith the contact electrode and the first metal layer, and the secondmetal layer having a minimum diameter that is smaller than a minimumdiameter of the upper face of the first metal layer but is larger than aminimum diameter of the lower face of the p-type nitride semiconductorlayer; and a protection film protecting an outer circumferential regionof the upper face of the n-type nitride semiconductor layer, side facesof the stack film, a region of the upper face of the second metal layerother than a region in contact with the p-type nitride semiconductorlayer, and a region of the upper face of the first metal layer otherthan a region in contact with the second metal layer.